Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats


Appendix II.

835

measurements are variously made. Mr. Thos. Turner loads a
diamond point till it just scratches, and the load measures the
hardness, while others have used points, knife edges or punches
of hard tool steel, and have severally measured volume, depth,
or length of the print. Prof. Unwin's method is more scientific,
for although he uses only a knife edge of tool steel, he has, by
plotting his experimental results, deduced the following formula :

Relative hardness = ~-

Where / = depth of indentation in inches.

p = pressure per inch width of knife edge.
n = 1*2.
His apparatus, Fig. 803, consists of a plunger A sliding in a

JL

socket B, and pressing on the piece of excessively hard tool steel
c placed on the specimen D. The whole is loaded in a testing
machine, as shewn by arrows, and both load and depth of im-
pression measured, the former being gradually increased, and time
allowed at each step. In no case must the specimen be stretched.

TABLE OF RELATIVE HARDNESSES.
(Measured by Prof. Unwin.)

Cast Steel        ............      554

Brass...............      233

Mild Steel       ...........      143

Aluminium (cast)        .........      103

Copper (annealed)       ...        ...        ...        62

Zinc (cast)        ...        ...        ...        ...       41

Lead (cast)       ............         4